Grayling Society
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Grayling Society Advisory Visit Afon Cain, Powys January, 2011 1 1.0 Introduction This report is the output of a site visit undertaken by Tim Jacklin of the Wild Trout Trust to the Afon Cain, Powys, on 28th January, 2011. Comments in this report are based on observations on the day of the site visit and discussions with Tony Bostock of the Severn Rivers Trust (SRT) and Richard Dearing, Catchment Officer with the Environment Agency (EA). Normal convention is applied throughout the report with respect to bank identification, i.e. the banks are designated left hand bank (LHB) or right hand bank (RHB) whilst looking downstream. 2.0 Catchment / Fishery Overview The confluence of the Nant Fyllon and Nant Alan about 2 km to the west of Llanfyllin forms the Afon Cain. The Cain flows eastwards for approximately 11 km through Llanfyllin and Llanfechain before joining the River Vyrnwy at Llansantffraid; the confluence with the Vyrnwy is a short distance (2 km) upstream of the Vyrnwy – Tanat confluence. Severn Rivers Trust are planning to include several stretches of the Cain in an angling passport scheme, aimed at providing access for visiting anglers and bringing economic and environmental benefit to the area. This advisory visit was carried out to assess the habitat on the various stretches and options for improvement. 3.0 Habitat Assessment Upstream of Llanfyllin The section of river (Figure 1) upstream of the town is owned by Mr. Baines of Bodfach Hall, with the part nearest town being leased by the Town Council. A small parcel of land near to Llanfyllin is being used to extend an existing car park and a wetland area has been created to take the drainage from the car park; this will buffer the run-off from the tarmac area and creates a pleasant riverside amenity area. Upstream from the wetland, the Council plan to include the leased section of river as a beat in the SRT angling passport scheme. 2 Figure 1 This section of river has a meandering plan-form with a pool and riffle sequence which provides a variety of depths and flow velocities – the basic building blocks of good habitat. These is a predominantly gravel substrate which is good for trout, grayling and salmon spawning. Some mature trees are present (mostly alder) and the root masses of these trees provide good bank stability and excellent cover for adult fish (Photo 1). Unfortunately the in-stream habitat is compromised by highly accelerated rates of bank erosion (Photos 2, 3). The outsides of meanders in many locations are being rapidly eroded by the process of block failure, where the river cuts away the toe of the bank and gravity causes the overhanging block of earth to collapse into the river. Although erosion (on the outside of bends) and deposition (on the inside) is a natural process which promotes the desirable natural pool-riffle sequence, accelerated rates of erosion cause poor in-stream habitat for fish. Resultant over-widening and shallowing of the river channel promotes the settling of fine sediments which compromise fish spawning and invertebrate 3 diversity (Photo 4). Also, the erosion is a significant source of fine sediment entering the watercourse and affecting downstream areas. Rapid loss of the important riparian (marginal) zone compromises habitat for fish, birds (including kingfishers and sand martins which nest in earth cliffs), mammals and invertebrates. There are two factors contributing to the accelerated rates of erosion on this section of river: • Over-grazing by high densities of sheep. The banks are closely cropped which removes bank-stabilising vegetation; this includes coarse grasses, sedges and reeds and also tree succession is prevented. As mature trees are lost (a process exacerbated by the increase erosion) none are coming through to replace them and provide much needed bank stability, shade and source of terrestrial invertebrates for fish food. • Increased coarse sediment supply. When the supply of cobble to pebble- sized sediment is increased within an upland catchment, this often leads to a “traffic jam” of sediment as the gradient of the river lessens; the supply of coarse sediment starts to exceed the rate the river can transport it downstream. The result is the deposition of large gravel shoals, and the river’s response is to find pathways around these, leading to accelerated rates of bank erosion. Some clear-felling of steep slopes in the Nant Alan valley was carried out in 2008 and may have resulted in increased coarse sediment supply; however the site was inspected during this visit and appears to be stable and being re-planted with native tree species. Recommendations 1. As a priority, grazing pressure should be reduced alongside the river. This could be achieved by changing the emphasis of the land use from livestock production to conservation grazing, for example by using low densities of traditional breed cattle. Alternatively, fencing out a generous buffer zone alongside the river would be required (5 metres minimum, preferably 10 m). Reduced grazing pressure is essential if any of the bank revetment techniques described below are to be successful. 2. Protect the rapidly eroding sections of river bank with soft revetment techniques, using materials like logs, brushwood, Christmas trees and 4 root wads. Advice and demonstrations of techniques are available from the Wild Trout Trust. For longer term protection of the outside of meanders it is recommended that fenced, ungrazed headlands are created and planted with native tree species. 3. Liaise with the owners and managers of forestry plantations in the upper catchment to find out the timescale for harvesting of existing blocks of forestry. Work with the forestry interests to devise a plan to minimise the release of sediments and nutrients during harvesting, and to promote sustainable future forestry practices, such as mixed species plantations with rotational harvesting rather than single- species stands and clear-felling. Photo 1 Good in-stream habitat – pool and riffle sequence, tree roots in the water and a gravel bed. 5 Photo 2 Block failure erosion on the right bank, possibly caused by a combination of over-grazing and increased coarse sediment supply from the upper catchment. Photo 3 Accelerated erosion – there are two sections of the concrete pipe (right bank) lying on the river bed here indicting the extent or recent bank loss. 6 Photo 4 Summer, low flow conditions – note the fine sediment accumulating on the gravel bed. (Photo: Tony Bostock) Photo 5 Some materials suitable for bank revetment are available on site. 7 Photo 6 River Manifold, Staffordshire where severely accelerated bank erosion was treated with brushwood revetment. Before... Photo 7 ...and after revetment and livestock fencing. Further guidance on techniques can be found in the WTT’s Upland Rivers Habitat Manual (www.wildtrout.org – publications link). 8 Green Hall Farm – Pentre Farm This section was walked upstream from Green Hall Farm (Figure 2). The plan-form of the river here is straighter than the previous section and there are fewer meanders and less of a pool-riffle sequence. It is likely that this section of river has been previously straightened (Photo 8). The bank stability here was much better than the previous section, with trees present along most of the banks; these were predominantly alder and willow. It is evident that some management of the banks and riparian trees is carried out on this section; areas of erosion have been tackled by placing tree trunks and in some cases rubble. In places, tree stems have been partially cut and laid (like hedge laying, Photo 9) parallel to the river bank – perhaps to provide a barrier to livestock accessing the river? This appears to be effective and whilst preferable to the unfettered access seen on the previous section, does tend to “hem in” the river. Figure 2 9 Because of the straightened nature of the channel here, deeper pool habitat which could hold adult fish is lacking which will restrict the value for angling. This could be improved by the retention of naturally occurring large woody debris within the channel, or by deliberate introduction of flow deflectors to create bed scour and deepen the channel in selected areas; the tree-laying technique could easily be adapted to achieve this by positioning the tops of the trees in the river channel. This provides excellent cover for all life stages of fish, improves invertebrate abundance and diversity and helps moderate water temperatures in hot weather. The upstream section of this beat is more open (Photo 10). Cut-off meanders and embankments here indicate that past straightening and bed lowering may have taken place. The effect on in-stream habitat is a wide, shallow channel with little adult fish habitat; also grazing and erosion of the banks has removed good marginal habitat greatly reducing the amount of juvenile fish habitat, particularly for trout. Occasional areas of more suitable habitat occur, usually where deeper water occurs on the outside of bends and in the vicinity of large woody debris which has been retained within the river channel (Photo 11). Discussion with the landowner and farmer, Mr. Peter Lewis, indicated that grayling were caught from this section of river in the past, but have not been recorded since a significant slurry pollution some years ago (late 1980s / early 1990s). Since the visit, EA survey results have been checked (Tony Bostock, pers. comm.) and grayling were recorded in low numbers on all the sections mentioned in this report except Llanfyllin (furthest upstream). A recent scientific study of grayling genetics in the UK (Dawnay et al., 2011) has shed light on the existing diversity and origins of the populations in different watercourses. The study suggests that grayling in the Severn catchment may constitute a genetically unique population.